How to fix mesh into a more uniform and simple version?
This is called retopologizing, it is something that's necessary for animatable objects or game assets, there are a lot of addons that help with retopologizing, these ease the process, not replace it.

There are some automatic solutions, a free one is Instant Meshes this will require further corrections.

I recommend doing it by hand, enable F2 and Bsurface addons, this can be quite time consuming so you should be happy with your mesh before doing your retopo, always create quads, game engines will always triangulate your model but originally it probably was 95% quads.